Tension remains high in eastern Ukraine, a day after hundreds of pro-Russian separatists attacked a border guard base in Luhansk city following reports of air raids by Ukraine's army on rebel positions.
The developments came as US President Barack Obama arrived in Warsaw, Poland, to begin a tour designed to reassure America's worried allies in Eastern Europe.
The pro-Russian separatists fired rocket-propelled grenades on Monday from the roof of a residential building adjoining the Luhansk base, prompting the reported deployment of air support by government forces.
At least five of the attackers were killed in fighting around the base, a spokesman for the guards told the AP news agency.
Al Jazeera's David Chater, reporting from Luhansk, said that the general opinion is that the offensive on the border base is a diversionary attack to allow more separatists to cross the border from Russia.
"Border guards are still stuck in the building, but the fighting is not going on. However, it can pick up anytime," he said.
Ukrainian soldiers had been trying to control a flow of separatist volunteers crossing the border from Russia.
Rebels massed near the base had earlier promised border officers that they would be unharmed if they surrendered and laid down their arms, Associated Press news agency reported.
The initial attack on the guard base sheltering about 70 soldiers was launched by about 100 rebels but the number of attackers swelled to 500 a few hours later.
In a separate incident on Monday, a blast at an administrative building in Luhansk held by separatists claimed several lives.
A health official for the Luhansk region told Russia's Interfax news agency that at least seven people had been confirmed dead in the blast.
Rebel forces from the self-proclaimed Luhansk People's Republic claimed that Ukrainian jets had responded to the attack on the border post by dropping cluster bombs on the administrative buildings they occupied in the city.
However, Ukrainian authorities denied carrying out a strike and said the blast was caused by misdirected rebel fire from a portable surface-to-air missile launcher.
A video showed an explosion, debris scattered around the building and men trying to move a body.
Al Jazeera's Chater said at least ten craters were created by the army's air strike.
"There is real anger happening here against the government," he said.
"Government claims that attack happened through missiles that came from rebels, which is clearly not. It is understandable from the scene"
Against this backdrop of violence, Russia accused Ukraine of committing "crimes against its own people".
Pro-Russian separatists, who have seized government and police buildings across eastern Ukraine, have mounted increasingly bold attacks on government-held checkpoints and garrisons in an attempt to seize weapons and ammunition from Ukrainian forces.
Russia called an emergency meeting of the UN Security Council on Monday to introduce a resolution calling for an immediate ceasefire in Ukraine.
"This draft resolution will also contain a requirement to immediately create humanitarian corridors that will help civilians leave hostility zones, should they wish to do so," Sergey Lavrov, Russia's foreign minister, said.
The move was immediately denounced by Ukraine and the US.
During the Poland leg of his four-day trip, Obama is expected to meet Ukraine's president-elect Petro Poroshenko, attend the Group of Seven summit in Brussels, meeting with the leaders of the largest economies of the world.
The US administration has spent months trying to isolate Putin and punish the Kremlin inner circle with sanctions over Ukraine.
The White House separately announced on Monday that Vice President Joseph Biden will travel this weekend to Ukraine's capital Kiev to further bolster US bilateral ties with Ukraine.